An ion mobility spectrometer typically comprises an ionization source, a drift cell, and an ion detector. Examples of an ion detector include a sampling plate, an electron multiplier, or a mass spectrometer. Ion mobility spectrometry separates ions in terms of their mobility in a drift/buffer gas by measuring the ion equilibrium drift velocity. When gaseous ions in the presence of the drift gas experience a constant electric field, they accelerate until the occurrence of a collision with a neutral atom or molecule within the drift gas. This acceleration and collision sequence is repeated continuously. Over time, this microscopic scenario averages the instantaneous velocities over the macroscopic dimensions of the drift tube resulting in the measurement of a constant ion velocity based upon ion size, charge and drift gas pressure. The ratio of the ion velocity to the magnitude of the electric field is defined as ion mobility. In other words, the ion drift velocity (νd) is proportional to the electric field strength (E), where the ion mobility K=νd/E is a function of ion volume/charge ratio. Thus IMS is a separation technique similar to mass spectrometry. IMS is generally known to have high sensitivity with moderate resolving power. Separation efficiency is compromised when “bands” of ions spread apart as opposed to arriving together at the end of the IM drift tube in a tight, well-defined spatial region.
The resolving power for a uniform or quasi-uniform ion mobility electric field increases as a square root of voltage applied along mobility cell. It would seem that there is not much freedom to increase the resolution. However, the situation may be improved if the ion drift in a gas flow is considered. Ions move against the gas flow only if the field is stronger than a certain value specific for the mobility of the ions. Ions with lower mobility may be stationary or even move in the negative direction (with the gas flow). Therefore, better ion separation can be expected where the time of this separation can be chosen suitable for specific applications and compatible with the time diagram of the ion detector operation. The problem is how to efficiently organize ion mobility separation using gas counter-flow. Most often an ion mobility separation is used with ion sources working under elevated pressure and the source pressure is often used when these ions are introduced into a mobility cell. There may be no gas counter-flow in such an application. On the other hand, drift gas counter-flow is inevitable when IMS is used for analysis of ions created in high vacuum ion sources such as a secondary ion source where secondary ions are created from a surface maintained in high vacuum and must then be moved against a counter-flow of gas into the ion mobility spectrometer. The main problem then is how to overcome the strong counter-flow and preserve ion throughput. It is quite natural to use a time varying electric field to gradually move ions from a pulsed ion formation region against the gas flow and into the IMS. Small ions need a relatively small field to overcome the gas flow without decomposing whereas larger ions can come to the entrance orifice later under the action of a stronger field. At the time of application of the larger field necessary to move the heavier ions, small ions are already inside the mobility cell and are not subjected to the strong field which would otherwise cause their fragmentation. Some separation of ions in addition to the usual mobility separation is achieved in this case, however, it is often rather small, because of the diffusion broadening during the initial ion cloud formation. The gas counter-flow itself is also useful because it prevents neutral species from getting into the mobility cell and degrading its performance by forming non-conductive deposits on the mobility cell electrodes. One of the crucial points for the present invention is the organization of the weaker counter-flow for the low pressure ion sources and purposeful creation of the counter-flow for high pressure ion sources for their interfacing with ion mobility cells.
The combination of an ion mobility spectrometer (IMS) with a mass spectrometer (MS) is well known in the art. In 1961, Barnes et al. were among the first to combine these two separation methods. Such instruments allow for separation and analysis of ions according to both their mobility and mass, which is often referred to as two-dimensional separation or two-dimensional analysis. Young et al. realized that an orthogonal time-of-flight mass spectrometer (oTOFMS) is the most preferred mass spectrometer type to be used in such combination because of its ability to detect simultaneously and very rapidly (e.g. with high scan rate) all masses emerging from the mobility spectrometer. The combination of a mobility spectrometer with an oTOFMS is referred to as an Ion Mobility-oTOFMS. This prior art instrument comprised means for ion generation, a mobility drift cell, an oTOFMS, and a small orifice for ion transmission from the mobility cell to the oTOFMS.
In 2003, Loboda (U.S. Pat. No. 6,630,662) described a method for improving ion mobility separation by balancing ion drift motions provided by the influence of DC electric field and counter-flow of the gas. Using this balance, ions are at first accumulated inside an ion guide, preferably an RF-ion guide, and then, by changing the electric field or gas flow, the ions are gradually eluted from the ion guide to the mass spectrometer. Such type of ion accumulation is restricted to collecting relatively small number of ions due to space-charge effect. It also has some limitation in ion mass-to-charge (m/z) range because RF-focusing for a given RF-voltage has decreasing efficiency for larger mass ions. Increasing RF-voltage in this case is limited due to the possibility of glow discharge at high voltages. For at least these reasons, this method has significant resolving power limitations, particularly for large mass ions. The time of ion accumulation and their storage in RF-ion guide should not be too long, otherwise ions would be partially lost due to diffusion into rods or walls confining the gas flow. The instrumental improvements disclosed below eliminate these drawbacks.
Use of MS as a detector enables separation based on mass-to-charge (m/z) ratio after the separation based on ion mobility. Shoff and Harden pioneered the use of Mobility-MS in a mode similar to tandem mass spectrometry (MS/MS). In this mode, the mobility spectrometer is used to isolate a parent ion and the mass spectrometer is used for the analysis of fragment ions (also called daughter ions), which are produced by fragmentation of parent ions. Below this specific technique of operating a Mobility-MS is referred to as Mobility/MS, or as Mobility-TOF if the mass spectrometer is a TOFMS-type instrument. Other prior art instruments and methods using sequential IMS/MS analysis have been described (see, e.g., McKight, et al. Phys. Rev., 1967, 164, 62; Young, et al., J. Chem. Phys., 1970, 53, 4295; U.S. Pat. Nos. 5,905,258 and 6,323,482 of Clemmer et al.; PCT WO 00/08456 of Guevremont) but none combine the instrumental improvements disclosed here. When coupled with soft ionization techniques and the sensitivity improvements obtained through the use of the drift cell systems disclosed herein, the IMS/MS systems and corresponding analytical methods of the present invention offer significant analytical advantages over the prior art, particularly for the analysis of macromolecular species, such as biomolecules.
One challenge when building a Mobility-MS system is to achieve high ion transmission from the mobility region into the MS region. It is at this interface that earlier uses of linear fields appear incongruous with the goal of maximizing ion throughput across the IMS/MS interface. The mobility section operates at typical pressures between 1 mTorr and 1000 Torr whereas the MS typically operates at pressures below 10−4 Torr. In order to maintain this difference in pressure it is necessary to restrict the cross-section of the exit orifice of the IM drift cell so that the region between the IM and the MS can be differentially pumped. Typically this orifice cross section is well below 1 mm2. Hence it is desirable to focus the ions into a narrow beam before they reach the interface. Another important property of ion beam coming into MS is the beam divergence, or the kinetic energy of ion motion in the plane orthogonal to the direction of their travel. This is the main factor responsible for the quality of mass spectra obtained in the orthogonal TOFMS. It is a subject of the present invention to achieve good ion beam properties by using a thin dielectric coating of the electrodes followed by controlled charging of this coating. It allows the use of a channel instead of an exit orifice with sharp edges for the IM drift cell and to form low divergent supersonic gas flow where ions could be significantly cooled to have an average energy of their side motion corresponding to a few ° K.
In 1997, Brittain, et al. (U.S. Pat. No. 5,633,497) described the coating of the interior surfaces of an ion trap or ionization chamber with an inert inorganic non-metallic insulator or semiconductor material for the passivation of the surfaces in order to minimize absorption, degradation or decomposition of a sample in contact with the surface
U.S. Pat. No. 6,600,155 to Andrien et al., teaches the coating of a surface in time-of-flight pulsing region with a dielectric film (among other types of films) for improving ion beam properties before orthogonal extraction of ions into the drift region of a time-of-flight mass spectrometer
Whitehouse (U.S. Pat. No. 6,707,037) proposed the extraction of ions of both signs from a MALDI target directly located inside gas-filled RF-multi-pole ion guide, to concentrate them along the axis of the guide, and send them in opposite directions under the influence of an axial electric field for subsequent mass analysis.
Park (US Patent Application No. 2004/0149902 A1) proposed the use of a multi-pole RF-ion guide to insert ions from a number of ion sources into analytical devices including mass spectrometers and mobility spectrometers. In 2002 Moini and Jiang in U.S. Pat. No. 6,465,776 described the insertion of ions from multiple electrospray capillaries through one quadrupole RF-ion guide where ion beams are mixed into TOFMS. However, multi-channel data recording was not disclosed.
U.S. Pat. No. 5,763,865 to Kaersdorf et al. disclosed a method and apparatus for quantitative non-resonant photoionization of neutrals. A time-of-flight mass spectrometer with novel ion mirror for separation of different ion beams is described. Eriksson, in U.S. Pat. No. 6,683,302 described an electrospray ion source wherein heating of droplets emerging from the electrospray capillary under the influence of strong electric field is provided by a microwave field between the spray tip and mass analyzer. U.S. Patent Application No. 20030226750 of Fenn suggests the use of AC voltage to produce charged droplets from the solution emerging from a conducting capillary instead of DC voltage used in conventional electro-spray ion sources. It was disclosed that the flow of the droplets for 60 Hz 5 kV AC voltage is similar to that of a conventional electrospray (ESI) ion source. In 2003 Ranasinghe et al. (US Patent Application No. 2003/0001090) proposed to split the liquid flow from some separation device into two approximately equal streams and direct them into two ion spray sources—the first one producing positive ions and the second one producing negative ions. Two TOFMS systems are used to record positive and negative ions separately.
In 2002, Berggren, et al. (US Patent Application No. 2002/0166961) described a charged droplet source for mass spectrometer with the focusing of droplets and ions using an aerodynamic lens. This lens is a sequence of coaxial apertures where the gas flow comes through orifices with decreasing diameter so that charged particles are focused to some extent.
In 2003 Cornish et al. (U.S. Pat. No. 6,580,070) suggested to combine several relatively simple coaxial TOFMS systems with MALDI or laser ablation ion sources. One array located inside a vacuum chamber provides high throughput analysis of several samples or one large sample in different points. In 2004 Hobbs, et al. (US Patent application No. 20040217279) described multianalyzer mass-spectrometer for the parallel analysis of multiple samples preferably coupled with fluid phase separation techniques
All of the above-referenced U.S. patents and published U.S. patent applications are incorporated by reference as though fully described herein.
Although much of the prior art resulted in improvements in ion focusing, separation and in ion throughput from ion source to the mobility cell and to the mass spectrometer in tandem instruments, there is room for additional improvement in all these directions. The inventors describe herein a concept and designs of multi-beam ion mobility and mass separations with multi-channel data recording which result in variety of instrumental embodiments to provide improved ion production from investigated samples, their separation and measurements.